PSI-driven cyclic electron flow (CEF-I) helps higher plants avoid severe heat damage. Gelidium amansii , a red seaweed used in the production of agar, inhabits subtidal rocks but can be found in the intertidal zone. The biological role of CEF-I is still unclear in this organism. Wild G . amansii was exposed to 30 ℃ heat stress for 12 h with continuous lighting. The results showed that treatment at 30 ℃ gradually decreased maximal PSⅡ photochemical efficiency ( F v / F m ), linear electron transfer rate, and activity of photosynthetic reaction center. Both the maximal photochemical efficiency under light ( F v \' / F m \' ) and maximum quantum yield of light-adapted PSⅡ( Φ PSⅡ) were maintained at a relatively stable level during the initial 6 h and then significantly decreased at 12 h. The up-regulated CEF-I helps to enhance proton gradient transfer across thylakoid membrane to protect oxygen-evolving complex against heat damage. Following the addition of a CEF-I inhibitor to plants, the F v / F m greatly decreased, suggesting that the CEF-I alleviates degree of photoinhibition caused by strong light. The results of measurement of antioxidant enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT), and the contents of H 2 O 2 and malonaldehyde (MDA) provided additional evidence that CEF-I plays a protective role to a certain extent for G . amansii to manage stress at 30 ℃. Therefore, it can be concluded that CEF-I enables G . amansii to survive in intertidal zones by protecting it from the heat damage caused by high temperature stress.